I've run into issues importing contacts using the exported file from the CRM being used. Those issues aside, it seems like the import process for contacts should include an undo option. In other word...
Status changed:
Reviewed by moderator
to Accepted
We are working to be able to see all your existing CSV imports, how many contacts were imported, and allow you to revert/undo by deleting the contacts if you made a mistake. This is slated for release at the end of this year/early next year.